Color appearance of a color image device must faithfully reproduce an original color. On the other hand, color appearance favored by people is also important and, especially, flesh color, sky blue, plant green, and the like must be color appearance favored by people. For example, when an object image is sensed by a digital camera and is recorded as digital image data, exposure is controlled upon image sensing, and the color balance, hue, saturation, and tone of color image data output from an image sensor are corrected, so as to obtain an image with appropriate brightness.
However, these processes associated with color conversion correct the brightness and colors of the entire image but cannot individually correct a color to be corrected. For this reason, when correction is applied to have a color to be corrected as a target color, other colors are influenced, and the entire image is not always corrected to favorable colors. For example, when a person in the shade is to be sensed, if the brightness level of his or her flesh is increased to a target brightness level, other regions which originally have favorable brightness levels become excessively bright. When the flesh color has a bluish tint or is excessively yellowish due to the influence of local illumination, if it is corrected to a favorable color, the colors of other regions which originally have favorable colors become unnatural.
In order to solve these problems, for example, “Image Signal Processing for Hardcopy” by Hiroaki Kotera, The Journal of the Institute of Television Engineers of Japan, Vol. 43, No. 11 (1989), pp. 1205-1212, introduces a selective color adjustment model. With this model, a color region which is to undergo color correction is designated, and the hue and saturation in the designated color region are corrected in a favorable direction.
According to the above reference, color adjustment is perceptually made using the hue correction amount and saturation scale. However, how to determine the hue correction amount and saturation scale is not explained in concrete terms. If color adjustment is made by setting the hue correction amount and saturation scale to be appropriate values, an image becomes unnatural due to too large a correction amount or an expected effect cannot be obtained due to too small a correction amount. A method of confirming a correction result image by displaying it on a display or printing it using a printer, and exploring an optimal hue correction amount and saturation scale while perceptually adjusting an image requires expert knowledge in adjustment or a huge number of times of trials.
Given an image which cannot be desirably corrected by conversion of the brightness or colors of the entire image, it is demanded to easily and effectively apply correction of hue and saturation in a desired color region of such an image.